The present invention relates in general to apparatus for the manufacture of asphalt-aggregate material, and in particular to continuous asphalt plants such as drum mix plants.
Drum mixing apparatus is known for use in the preparation of asphalt-aggregate paving compositions. It is generally recognized that drum mix asphalt plants provide certain advantages in comparison with other types of asphalt plants, including the economy of continuous-flow operation and relative portability for transportation between job locations. A typical drum mix asphalt plant includes a rotating mixing drum in which aggregate material and asphalt material are separately added and mixed together while being heated to provide the desired asphalt-aggregate mixture. The contents of the mixing drum are heated by a fuel burner located at one end of the drum, and the asphalt-aggregate mixture is removed from the other end of the drum for immediate use or temporary storage in a manner known to those skilled in the art.
The introduction and agitation of aggregate material in the mixing drum of a drum mix asphalt plant, combined with the heating of the aggregate material, produces a substantial volume of dust and other relatively fine airborne particulate mineral matter which emerges from the mixing drum. The amount of such airborne particulate matter may exceed prevailing standards of allowable air pollution, and so drum mix asphalt plants are increasingly required to operate in conjunction with suitable apparatus for reducing the particulate matter to an acceptable maximum level. While fiber filter dust collection systems and venturi scrubber systems are frequently used to remove dust and similar fine particulate matter produced by heating and agitating aggregate material alone, the amount of smoke produced by recycling used asphalt-aggregate material in drum mix asphalt plants is potentially more than can be practically removed by fiber filter and venturi dust collection systems with such plants.
Smoke is also produced in drum mix asphalt plants by so-called "flashing" of the liquid asphalt material upon exposure to the elevated temperature within the mixing drum, and is caused by evaporation of light end hydrocarbons of the asphalt material to produce blue smoke emissions. These hydrocarbon smoke emissions will rapidly and permanently clog the porous filter media within the typical so-called "baghouse" fiber filter dust collection system rendering the baghouse inoperative until the filter bags are replaced. Replacement of fuel-clogged filter bags is an expensive expedient and renders the baghouse (and therefore the entire drum mix asphalt plant) inoperative for a period of time.
The recycling of old asphalt-aggregate material such as roadway material is very desirable from an economic viewpoint. If a roadway can be torn up and used to replace fifty to one hundred percent of the new virgin aggregate and liquid asphalt otherwise needed to re-surface the roadway, a substantial raw materials cost saving may be realized. However, the use of old asphalt-aggregate material aggravates the smoking problem due to the fact that the aggregate is already coated with asphalt when it is subjected to high temperatures in a drum mix plant. In order to eliminate or minimize the smoking of material being recycled in drum mix plants, several systems have been proposed.
In one prior art method the used asphalt-aggregate material is introduced, along with virgin aggregate, at or near the fuel burner used to heat the virgin aggregate material at the upper end of the drum mixer, which is generally inclined so that the material within the drum will gradually move toward the removal point as the drum rotates. This method, however, causes the used asphalt-aggregate material to smoke excessively, and it has been found that a plant using this method cannot be operated within current air pollution standards.
Another prior art system utilizes a drum within a drum whereupon the used asphalt-aggregate material is introduced into the upper end of an outer drum and virgin aggregate is introduced into an inner drum which contains the heat source. After the virgin aggregate has been heated, it emerges from the inner drum for mixing with the material being recycled in the outer drum. A disadvantage of this prior art system is that it has a small capacity for production using only virgin materials. Thus, such a plant is inefficient for use where old asphalt material is not available.
A third prior art system has been used solely for recycling used asphalt material. The interior of the drum mixer is heated by circulating heated fluid through tubing within the drum, and the used asphalt material is tossed about the drum outside the tubes. It has been found that asphalt-aggregate material when heated tends to adhere to the tubes and block off the mixing drum. Also, a second complete drum mix plant is required if virgin materials are to be used.
A fourth prior art system is shown in U.S. Pat. No. 3,999,743 which discloses a system for crushing and sorting used asphalt aggregate material by size, introducing coarse particles into the flame of the fuel burner, and introducing smaller particles at locations more distant from the burner. The particles are admitted directly into the drum through the cylindrical wall of the drum as it rotates by way of a plurality of complex feeding mechanisms. A disadvantage of such a system is that the burner temperature must be held low enough to prevent burning the coarse particles, resulting in a lower temperature in the lower end of the drum and therefore a lower production rate since more time is required for sufficient heating of the material. Further disadvantages are the lack of provision for the simultaneous use of virgin raw materials with used material and the expensive nature of the complex side-delivery system disclosed.
Another prior system introduces the used material and the virgin material at the upper end of the rotating drum adjacent a burner. A burner shield projecting into the drum prevents the flame of the burner from directly contacting the asphalt material. An excess amount of air is pulled into the combustion area of the burner to cool the combustion gases to such an extent that the temperature of the gases upon contact with the asphalt material does not produce smoking of the asphalt material. Sufficient heating of the asphalt mix material in the drum is provided by regulating the burner in response to the temperature of the asphalt material exiting the drum. A disadvantage of this system is that it inherently uses fuel for the burner inefficiently since an excess of cool air is used to lower the temperature of the combustion gases. Furthermore, if the exit temperature of the mix is too low, the burner temperature may be increased automatically to a point where smoking of the used material will occur unless still more excess air is introduced.